Recently, as massive MIMO antenna systems can significantly improve the spectrum efficiency and energy efficiency, massive MIMO antenna systems have been considered as a part of critical 5G technology in the future, and have attracted wide attention from academia and industry.
In the prior art, in order to make full use of diversity gain and multiplexing gain of a massive MIMO antenna system, a base station (BS) needs to be aware of the channel state of a channel between the base station and a user equipment (UE). In a Time Division Duplex (TDD) system, the channel state of a downlink channel between a BS and a UE can be obtained by using channel reciprocity. Therefore, a large number of documents in the current are focused on TDD massive MIMO antenna systems. However, TDD systems also face problems such as pollution of pilot signal (also referred to as training sequence, reference sequence, etc), and it is difficult for TDD systems to support high-speed mobile communication scenarios. On the other hand, most current cellular mobile communication systems operate in the Frequency Division Duplex (FDD) mode, and therefore the FDD mode is bound to be preserved in the evolution to the 5G standard.